The invention relates to an electrically tuneable optical filter which may be used, in particular, for spatially separating frequency components in an input beam of radiation, at microwave or optical frequencies. More particularly, the device may be used as a staring spectrum analyser or as a wavelength division multiplexer and demultiplexer.
There are two approaches to making a spectrum analyser. In one case, the input signal to be analysed may be passed through a tuneable filter with is scanned through the required frequency range. The transmission against the frequency of transmission of the filter is then measured to give the spectrum. Such systems are known as scanning spectrum analysers. Alternatively, the input signal may be split into a number of identical, lower power signals, each of which is then passed through a different filter from a set of equally spaced filters. The output powers of the filter set give the spectrum required. Such systems are known as staring spectrum analysers.
Conventional RF spectrum analysers, as used in the laboratory, are scanning spectrum analysers. This is because a scanning spectrum analyser can cover a wide range of frequencies with a high resolution and can easily be reconfigured as required. However, a scanning spectrum analyser is only useful for measuring input signals which are not changing rapidly, as they can only xe2x80x9clookxe2x80x9d at one frequency at a time. If, for example, the input signals contain fast pulses, the scanned spectrum analyser could easily miss pulses of some frequencies if it is not looking at the right frequency when the pulse arrives.
A staring spectrum analyser overcomes this problem. However, it is more difficult to make than a scanned spectrum analyser, especially if the number of frequency channels is high. As splitters tend to be narrow band components, it is difficult to split a broadband electrical RF signal many ways without severe distortion. Also, RF filters have to delay the signal by a time proportional to the inverse of the filter bandwidth, and this makes such components very large and difficult to make with low enough loss to achieve a resolution below around 100 MHz using conventional techniques.
Optical methods can be used to make both scanned and staring spectrum analysers. An example of a scanned optical spectrum analyser is a scanning Fabry Perot interferometer, which comprises two parallel (or confocal) plates which are moved towards and away from each other (usually using a sawtooth drive voltage). The output intensity plotted against time gives the optical spectrum [xe2x80x9cIntroduction to optical electronicsxe2x80x9d, A. Yariv (Holt Reinhart and Winston, 1976)].
A grating spectrometer is an example of an optical staring spectrum analyser. This works by splitting the input beam into many hundreds of beams, changing the phase of each beam by an amount which depends linearly on its position (using the grating) and recombining all of the phase shifted beams on an output detector array. Because of the phase shifts, different optical frequencies recombine in phase at different places in the detector array.
Another type of staring optical spectrum analyser is an acoustic-optic device in which the signal to be analysed is used to drive an acoustic-optic transducer which launches an acoustic wave into a transparent piezoelectric and electro-optic material (e.g. lithium niobate). The acoustic waves can set up refractive index waves in such materials which diffract a light beam passing through them by an amount directly proportional to the RF frequency. In practice, this type of spectrum analyser can give very high resolution, mainly because acoustic waves travel much more slowly than electromagnetic waves, allowing longer delays to be achieved in short devices. However, they tend to be limited to frequencies below a few GHz because of acoustic losses.
Various optical waveguide versions of low resolution optical spectrometers have been demonstrated, usually for combining (multiplexing) or splitting (demultiplexing) a number of different wavelengths on one fibre. These are passive devices, however, rather than active devices, which are made by accurate lithography and design to control the optical phase shifts. However, lithographic inaccuracies are inevitable and this limits the resolution which may be achieved with such systems.
UK 2 269 678 A is in the field of the present invention. It describes an interferometric tuneable filter formed on a semiconductor substrate on which a waveguide is split into a plurality of branches of equal length. Each branch has electrically controllable amplitude and phase control elements for modulating the amplitude and phase of light transmitted through the branches. The filter has the function of selecting a predetermined wavelength light signal from a plurality of multiplexed light signals. The light transmitted through each branch is recombined to provide a single device output. The device described in UK 2 269 678 A therefore has the disadvantage that only one wavelength is output from the device, the others being lost in the substrate. The device is therefore not suitable for use as a spectrum analyser or in applications where multiple outputs of different wavelength are required.
According to the present invention, a device for spatially separating components of frequency in a primary radiation beam comprises;
means for separating the primary radiation beam into a plurality of secondary radiation beams each having a phase, xcfx86,
a plurality of electrically biasable waveguides forming a waveguide array, each for transmitting a secondary radiation beam to an output, wherein each waveguide has an associated optical delay line having a corresponding optical delay time, wherein each of the optical delay times is different,
means for applying a variable electric field across each of the waveguides such that the phase, xcfx86, of the secondary radiation beams transmitted through each of the waveguides may be varied by varying the electric field,
whereby the secondary radiation beams output from each of the waveguides interfere in a propagation region with a secondary radiation beam output from at least one of the other waveguides to form an interference pattern comprising one or more maximum at various positions in the propagation region.
Preferably, the device provides at least two outputs.
The device may also comprise means for applying RF modulation to the primary radiation beam. The device may therefore be used as an optical staring spectrum analyser or an RF spectrum analyser and has an advantage over scanning spectrum analysers where signals of some frequencies may be missed if the device is not scanning at the right frequency when a pulse of radiation arrives. Furthermore, the device can be used to scan both optical and microwave frequencies by use of the RF modulation means and may be actively controlled in use by varying the electric fields applied across one or more of the electrically biasable waveguides. Because the phase of radiation transmitted through each of the waveguides may be varied, any inaccuracies in the design may therefore be corrected for in use, by varying the applied electric fields. This provides an advantage over passive devices used for multiplexing and demultiplexing beams.
In a preferred embodiment, each adjacent pair of waveguide outputs are spaced apart by an amount proportional to the optical time delay difference between the corresponding adjacent pair of waveguides. This has the advantage that different intensity maxima corresponding to different optical frequencies occur at well defined angles in the propagation region and the angular difference between the maxima for two different frequency components is substantially proportional to the difference in frequency between the two frequency components.
In a further preferred embodiment, the waveguides may have a substantially linear variation in optical time delay across the waveguide array.
Typically, the optical time delay difference across the waveguide array is at least 100 picoseconds and may be at least 10 nanoseconds.
The propagation region may be a region of free space or may be a slab waveguide. A slab waveguide may be used to advantage as it confines the secondary radiation beams, for example to a chip.
The electrically biasable waveguides may be group III-V semiconductor waveguides. For example, GaAs or InP/InGaAsP waveguides may be used.
The electrically biasable waveguides may be formed in one with the optical delay lines and the device may be formed on a single chip. Alternatively, each of the electrically biasable waveguides may comprise a electrically biasable waveguide and an optical fibre delay length.
The means for separating the primary radiation beam into a plurality of secondary radiation beams is a multi mode interference splitter.
Each of the electrically biasable waveguides may have independent means for varying the electric field across the electrically biasable waveguide portion. For example, each electrically biasable waveguide may have an independent variable voltage supply.
The device may also comprise one or more output waveguide located within the propagation region, such that the or each output waveguide may receive a secondary radiation beam of selected frequency output from one or more waveguide.
In addition, or alternatively, the device may also comprise an input laser having a laser cavity, wherein the electrically biasable waveguides form part of the laser cavity to enable selective wavelength tuning of the laser by variation of the electric fields applied across each of the waveguides.
According to a second aspect of the invention, a method for spatially separating components of frequency in a primary radiation beam comprises the steps of;
(i) separating the primary radiation beam into a plurality of secondary radiation beams each having a phase, xcfx86,
(ii) transmitting each of the secondary radiation beams through one of a plurality of electrically biasable waveguide forming a waveguide array, wherein each waveguide has an associated optical delay line having a corresponding optical delay time, wherein each of the optical delay times is different,
(iii) applying a variable electric field across each of the electrically biasable waveguides and
(iv) varying the electric field across each of the electrically biasable waveguides so that the phase, xcfx86, of the secondary radiation beams transmitted through each of the electrically biasable waveguides may be varied,
(v) outputting the secondary radiation beams into a propagation region within which they may interfere with one or more of the other secondary radiation beams and
(v) forming an interference pattern in the propagation region comprising one or more maximum at various positions.
The method may comprise the further step of;
(vi) deducing the frequency components in the primary input beam from the positions of the or each maximum in the propagation region.